Page:Scheme - An interpreter for extended lambda calculus.djvu/21

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Sussman and Steele

December 22, 1975

20

Substitution Semantics and Styles

(DEFINE FACT
   (LAMBDA (N C)
       (== N 0
           (LAMBDA () (C 1))
           (LAMBDA ()
               (-- N 1
                   (LAMBDA (M)
                       (FACT M (LAMBDA (A) (** A N C)))))))))

We can see here that no functional application returns a value in a computation of factorial in this situation. We believe that functional usage, where applicable (pun intended), is more perspicuous than continuation-passing. We shall therefore use functional primitives such as * rather than ** wherever possible, keeping in mind that we could use ** instead if we wished.